If you feel tired, headachy, or foggy in your first week of keto, you're not alone — and you're probably not sick. This bundle of symptoms is so common it has a nickname: the keto flu. It isn't a real flu and it isn't an infection. It's your body reacting to a sudden drop in carbs.

The good news: it's temporary, it's manageable, and once you understand what's happening, the fixes are simple.

What the keto flu feels like

A peer-reviewed review of people starting keto found these symptoms most often (scoping review, 2025):

  • Headache
  • Tiredness and low energy
  • Dizziness or lightheadedness
  • Muscle cramps
  • Constipation
  • Nausea
  • Brain fog and irritability
  • "Keto breath" — a slightly fruity smell

Not everyone gets all of them, and severity varies a lot from person to person.

Why it happens (the simple science)

When you cut carbs, two things happen at once.

1. You lose water and salt. Carbs make your body hold on to sodium. When carbs drop, your insulin level falls, and your kidneys start flushing out sodium — and water goes with it (review). That's why you drop several pounds in the first few days: it's mostly water, not fat. But losing sodium, potassium, and magnesium is what triggers the headaches, fatigue, dizziness, and cramps.

2. Your fuel is switching over. For a couple of days your body has less glucose to burn but isn't yet making many ketones. During this short window you can feel low on energy (review).

The Cleveland Clinic sums it up simply: keto flu is "really just a shift in fluid status" (Cleveland Clinic).

How long it lasts

Symptoms usually start 2–3 days after you cut carbs and fade within 2–4 weeks for most people, often with little or no intervention (review). For many it's gone within a week.

What actually helps

Replace your salt

This is the single most useful step. Add salt to your food, or sip broth or bouillon. The research review mentions 1–2 cups of broth a day (about 1–2 grams of extra sodium) as a common recommendation (review).

Get potassium and magnesium

Eat potassium- and magnesium-rich keto foods: leafy greens, avocado, salmon, nuts, and seeds. Some people add a magnesium supplement if cramps are bad — check with a pharmacist or doctor first.

Drink enough water

Aim for plenty of fluids — the review suggests over 2 litres a day during the transition.

Eat enough fiber

Constipation is common. Non-starchy vegetables, nuts, and seeds (around 15–20 g of fiber a day) help (review).

Ease in instead of going cold turkey

You don't have to slash carbs overnight. Lowering them over a week can make the transition gentler.

An honest note

Most of these tips are sensible and low-risk, but it's worth knowing that strong clinical proof that any single remedy "cures" keto flu is still limited — the review's authors point out that no study has directly tied the proposed causes to symptom relief. The most reliable fact is that the symptoms are temporary and usually resolve on their own.

When to talk to a doctor

See a healthcare professional if symptoms are severe, don't improve after a few weeks, or include fainting, a racing heart, or confusion. Important: loading up on sodium is not safe for everyone — if you have high blood pressure, heart disease, or kidney disease, talk to your doctor before adding salt or electrolyte supplements.

New to all this? Start with What is the ketogenic diet? and learn the signs that you've reached ketosis.

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